1. Field of the Invention
The present invention relates to an LED drive circuit that drives an LED (light emitting diode).
2. Description of the Related Art
Featuring low power consumption and a long life, etc., the LED finds applications in increasingly wide areas, not only display apparatuses but also lighting equipment, etc. In the lighting equipment, a plurality of LEDs are used to obtain desired illumination.
The LED shortens its life if receiving a current exceeding a rated current value. Therefore, the LED needs to be driven with a constant current or provided with current limitation so that no current above a predetermined level is passed therethrough.
Mostly, general lighting equipment uses a commercially available AC 100 V power source. Thus, given that LED lighting equipment is used to take place of an incandescent lamp, etc., it is preferable that, like the general lighting equipment, the LED lighting equipment be so arranged as to use a commercially available AC 100 V power source as well.
An example of a configuration of a conventional LED drive circuit that can be used in an LED lighting equipment is shown in FIG. 20 (see JP-A-2000-260578). A conventional LED drive circuit shown in FIG. 20 drives an LED with a constant current, and is provided with: a bridge diode 2; a resistor R20_2; and a constant current circuit A20. The constant current circuit A20 is provided with: an NPN transistor Q20; a resistor R20_1; and a Zener diode ZD20.
The bridge diode 2 is, on an input side thereof, connected to a commercially available AC 100 V power source 1 and is, on an output side thereof, connected to an LED module 3 which is a module having a plurality of LEDs connected in series; that is, the bridge diode 2 has a positive polarity output terminal from which the LED module 3, the NPN transistor Q20, and the resistor R20_1 in this order are connected in series. One end of the resistor R20_2 is connected to a connection point where the bridge diode 2 and the LED module 3 are connected together. A base of the NPN transistor Q20 is connected to the other end of the resistor R2_2 and a cathode of the Zener diode ZD20. An anode of the Zener diode ZD20 is connected to the connection point where the resistor R20_1 and the bridge diode 2 are connected together.
With this configuration, an AC voltage outputted from the commercially available AC 100 V power source 1 is fully rectified by the bridge diode 2, and thereby a pulsating voltage having its peak of approximately 141 V is obtained. In the constant current circuit A20, a base potential of the NPN transistor Q20 is clamped to be constant by a Zener voltage VZ of the Zener diode ZD20. Thus, let the voltage between the base and an emitter of the NPN transistor Q20 be VBEQ20, voltages of the resistor R20_1 at both ends thereof are expressed by (VZ−VBEQ20), and let the resistance value of the resistor R20_1 be R20—1, a current passing through the resistor R20_1 is expressed by (VZ−VBEQ20)/R20—1, which means that the current passing through the resistor R20_1 is constant. That is, a current passing through the LED module 3 is constant as expressed by (VZ−VBEQ20)/R20—1.
Typically, the voltage of the Zener diode exhibits positive temperature characteristics (i.e., as a temperature rises, the voltage is increased), and the voltage between the base and emitter of the transistor exhibits negative temperature characteristics (i.e., as a temperature rises, the voltage is decreased), and the resistor exhibits positive temperature characteristics (i.e., as a temperature rises, the resistance value is increased). Accordingly, the constant current circuit A1 exhibits positive temperature characteristics (i.e., as a temperature rises, the constant current value is increased). Thus, with the conventional LED drive circuit shown in FIG. 20, a rise in temperature possibly leads to a current falling beyond a predetermined value injected into the LEDs.
In the conventional LED drive circuit shown in FIG. 20, neither component nor circuit for over-temperature protection is provided for a case where the LED module 3 or the LED drive circuit itself experiences a high rise in temperature. Thus, there is a possibility that the LED module 3 or the LED drive circuit is broken at worst if its ambient temperature rises abnormally high, or if a short circuit occurs between an anode terminal and a cathode terminal of the LED module 3. Moreover, if an excessive load is imposed, an incandescent lamp will fail in an open mode with its filament cut, and the LED module 3 and the LED drive circuit, each of which is formed with a semiconductor component, will fail in a short mode. Therefore, the LED drive circuit has to be so made as not to smoke or to be burned even in the worst case where a short circuit occurs to the LED module 3 or any component forming the LED drive circuit.
There are various protection devices and temperature sensing devices generally used, such as POSISTOR ((registered trademark) manufactured by Murata Manufacturing Co., Ltd., Japan)). However, they all have a low voltage rating and a low power rating, and are limited in applications in a case where no constant DC voltage is generated, and instead a commercial power source is directly connected to the LED drive circuit to drive the LEDs with a pulsating current.